51. N-Queens

vector<vector<string> >ans;
    void solve(int n,vector<string>&board,bool up[],bool down[],bool line[],int col=0)
    {
        if(col == n)
        {
            ans.push_back(board);
            return;
        }

        for(int row=0;row<n;row++)
        {
            if(line[row] == false && down[row+col] == false && up[n-1+col-row] == false)
            {
                board[row][col] = 'Q';
                
                line[row] = true;
                down[row+col] = true; 
                up[n-1+col-row] = true;
                
                solve(n,board,up,down,line,col+1);
                
                line[row] = false;
                down[row+col] = false; 
                up[n-1+col-row] = false;
                
                board[row][col] = '.';
            }
        }
    }
        
    vector<vector<string>> solveNQueens(int n) 
    {
        string str(n,'.');
        vector<string>board(n);
        
        for(int i=0;i<n;i++)
        board[i] = str;

        bool row[n];
        fill(row,row+n,false);
        bool down[2*n-1];
        fill(down,down+(2*n-1),false);
        bool up[2*n - 1];
        fill(up,up+(2*n-1),false);
        
        solve(n,board,up,down,row);

        return ans;
    }